The long-term goal of this research is to understand the chemical, biochemical, and biological aspects of thyroid hormone and its bioactive metabolites such that safer and more effective therapeutic agents can be developed that act at targets of the thyroid hormone endocrine system. We recently discovered a novel class of endogenous compounds called thyronamines that are chemical derivatives of thyroxine (T4). 3- Iodothyronamine (T1AM), the most active member identified to date of this class, has no affinity for the nuclear thyroid hormone receptors TR1 and TR2, but nevertheless has unique and potentially therapeutically useful biological actions. Single doses of T1AM rapidly induce hypothermia, bradycardia, and hyperglycemia in rodents. In addition, T1AM induces a profound fueling shift away from carbohydrates and toward fat burning in Siberian hamsters, a hibernating rodent, as well as mice. We recently demonstrated that chronic administration of low doses of T1AM inhibit feeding behavior in obese mice; i.e. like leptin, T1AM appears to act as an endogenous anorectic agent. In rodents and humans endogenous T1AM is found in circulation and tissues at levels that are similar to that of T3, one of the principle bioactive metabolites of T4. Similarly to T4, circulating T1AM is tightly bound to serum binding proteins and these binding proteins are different from those used to transport T4. In human serum, T1AM binds specifically to apoB-100, the primary lipoprotein of low density lipoprotein (LDL) particles, suggesting a potential role of T1AM in modulating cholesterol homeostasis. T1AM has very unusual pharmacokinetic properties for a biogenic primary phenethylamine. The plasma half- life of T1AM is 5.5 hr in mice whereas half-lives of 1-2 min are the norm for chemically similar biogenic amines such as serotonin and dopamine. In addition, the volume of distribution of T1AM is very high suggesting that T1AM distributes widely from circulation into all tissues including poorly perfused tissues such as fat and muscle. Further consistent with this is the finding that T1AM is robustly transported into a variety of cell types, including cells derived from fat and skeletal muscle, and this transport is mechanism is highly selective for T1AM and occurs by a mechanism that does not involve a member of the biogenic amine reuptake transporter family. Understanding these unusual biochemical properties of T1AM and the underlying physiology they reveal is the direct goal of this grant proposal. The research plan is comprised of the following Specific Aims: (1) . Isolate and Characterize T1AM binding proteins in mouse and rat serum; (2) Determine whether T1AM affects cholesterol uptake and metabolism in vivo; (3) Discover and characterize conjugated metabolites of T1AM.; (4) Determine the mechanism of intracellular uptake of T1AM and search for T1AM target proteins.